Bald Eagles Wintering in the Southern Gulf Islands, British Columbia

BALD EAGLES WINTERING IN THE SOUTHERN GULF ISLANDS, BRITISH COLUMBIA DAVID HANCOCK HIS study represents the first attempt to census regularly the wintering T Bald Eagles Haliaeetus Zeucocephdus in the southern Gulf Islands, Brit- ish Columbia. Data were also collected on the feeding and hunting habits of the wintering eagles. In 1961 the National Audubon Society initiated its 5-year Continental Bald Eagle Study. This North American survey was prompted largely by the alarming decrease in numbers of the eastern Bald Eagle in recent years. The studies of Broley (1947)) Howell (1962)) and others have amply pointed out the plight of this great bird in the eastern United States. More recently Southern (1963) conducted a survey of the Bald Eagles wintering along the Mississippi River in northwestern Illinois. No similar studies have been conducted on the west coast prior to the Continental Bald Eagle project, or to my study. Murie (1940 and 1959) described the food habits of the eagles of the Aleutian Islands, and Dixon (1909) gives a brief history of the Alaskan birds. Brooks (1922) and Munro (1938) p resented a few of their observations on the feeding and hunting habits of Bald Eagles in British Columbia. Less important notes on individual sightings, nestings, and feeding incidents complete the literature on this species on the west coast. METHOD Fifteen aerial counts, involving 24 hours and 15 minutes of flying time, were conducted over the area (Fig. 1) from 26 September 1962 to IS April 1963. In addition, 130 hours were spent on ground observations. The study area (shown in Fig. 1) is encompassed by 48”33 ’ and 48”45 ’ north latitude and 123”30 ’ and 123”15 ’ west longitude. This represents those southern Gulf Islands lying east of the Saanich Peninsula, Vancouver Island, B. C., and west of the United States border. About 50 square miles of land and water were covered. The two-member flight crew consisted of an observer-recorder, and myself as pilot-observer. A two-seater seaplane (Luscombe) was used. The flight speed varied from 75-100 mph. The aircraft was flown around the shoreline of each island at between 75 to 175 feet. In addition, the larger islands, Moresby, Portland, Sidney, Coal, and Piers Islands, were contoured at quarter-mile intervals-at about 100 feet above the treetops. The study transect represented that area extendin g outward approximately 250 yards from either side of the airplane. B ’rr d s were recorded in approximate locations on 111 David BALD EAGLES IN BRITISH COLUMBIA Hancock 113 work maps, according to age, and whether they were observed sitting or flying. During February four additional flights (9 hours flying time) were carried out to assess the wintering eagle populations of some of the remaining Gulf Islands. On these flights most of the major shorelines were covered, but many of the small bays and much of the inland area were overlooked. Table 1 lists the major islands covered and gives the actual number of birds seen, along with my estimation of how many birds were actually present. This estimate allows for both the areas that were not flown and for the underestima- tion due to the difficulty in spotting the birds. This area encompassed about 500 square miles and is referred to as the extended study area. Table 1 also shows the results from a census flight over San Juan Island, U.S.A. My estimate of total birds present in the San Juan group unfortunately is based on very little data. Breeding density and productivity will be dealt with in a later paper. There is undoubtedly some inaccuracy in determining the age class ratios (adults to immatures) . The white head and the black body of the adult birds make their presence more conspicuous than the duller and more mottled immatures. In addition, ground and aerial observations suggest that immatures more often perch in the lower limbs of trees than do the adults, and are thus less easily viewed from the air. It therefore seems likely that more immatures are missed than adults, but that flying adults and flying immatures are spotted with equal ease. Of the birds observed flying, 19 were adults and 26 were immatures. Therefore, if we assume the group of birds observed sitting and the group observed flyin, w are both represented equally by the ratio of adults to immatures in the total population, we can conclude first that adults and immatures represent approximately $0 and 60% of the population respectively. Second, assuming for the moment that the sitting adult population (219) is correct, we would then expect to have observed about 300 sitting immatures. Only 110 sitting immatures were observed. On this basis we can say we observed only about 40% of the immatures present. This line of reasoning has two major drawbacks. First, it assumes that the time spent sitting and flying by each age-group is proportional. Second, it assumes that the adult count is correct, when, in fact, some adults were probably overlooked. This would have made the error even larger. I doubt that the bias is as large as suggested by the above reasoning. Errors due to duplication are considered negligible. As a result of more recent aerial and water counts I have reached the conclusion that my censusing technique for the transect underestimates the adult age-class by not more than lO-15% and the immature class by about 2&35%. Wh i 1e my method underestimates the actual eagle THE WILSON BULLETIN June1964 114 Vol. 76, No. 2 TABLE 1 PEAK COUNTS AND FORECASTS OF TOTAL BALD EAGLES WINTERING IN THE GULF ISLANDS, B.C., AND SAN JUAN ISLANDS, WASH. 196%63* Peak counts Foi-ecast.S Date Ad. Imm. Total Ad. Imm. Total GULF ISLANDS, B.C. Study area 12 Feb. 29 33 62 33 45 78 Saltspring 9-16 Feb. 10 5 15 25 20 45 Prevost 9 Feb. 8 1 9 13 8 21 North Pender 20 Feb. 4 1 5 10 8 18 South Pender 20 Feb. 2 0 2 4 4 8 Saturna 20 Feb. 11 6 17 22 20 42 Mayne 20 Feb. 8 5 13 13 12 25 Galiano 20 Feb. 19 6 25 31 20 51 Kuper 7 7 14 Thetis _ 8 8 16 Valdes _ 10 10 20 Gabriola _ - 10 10 20 Approximate total of estimated wintering population: 175 175 350 Population density I- 0.7 Bald Eagle per square mile SAN JUAN ISLANDS, WASH., U.S.A. San Juan 7 Feb. 22 13 35 30 30 60 Remaining islands 7 Feb. - _ 50 50 100 Approximate total of estimated wintering population: 75 75 150 * Totals are given for each major island and this includes the small nearby islands. population, the relative seasonal changes in abundance are reliable since the censusing technique has remained constant throughout the study. SEASONAL MOVEMENTS The change in abundance is shown by Fig. 2. Several population movement patterns have become apparent. At the onset of the project, on 26 September 1962, no eagles were present in the study area. The first bird was observed in the area by Darcy Goyette (pers. comm.) at Piers Island on 21 October. My count of 24 October found nine adults present. A local naturalist, Jack Todd (pers. comm.), spent from 12-18 October on Sidney Island and did not see an eagle. Therefore, I feel that the date of the eagles ’ arrival on the study area is about the third week in October. Not only were the first birds to arrive adults, but these adults in many cases appeared paired and were located in the vicinity of nest sites. It is my opinion that the first birds to arrive on the “winter territory” were the breeding adults of that territory. During the months of November and December the eagle population remained relatively stable (lo-17 adults and O-2 immatures). The eight nest sites David BALD EAGLES IN BRITISH COLUMBIA Hancock 115 WINTERING BALD EAGLES OF SOUTHERN GULF ISLANDS, B.C. 6 0 .- TOIAL EAGLES - ADULTS __-_ I MM*T”RE* . ...‘ ...‘ ..‘ FIG. 2. Wintering Bald Eagles of Southern Gulf Islands, B. C. known in the study area could nearly account for all these adult birds. It might be pointed out that in most cases it was the presence of the adult birds near the nests that made the nests conspicuous. Between 31 December 1962 and 16 January 1963, the population increased from 18 to 59 birds. This new group of birds was composed nearly equally of adults and immatures. This high population of birds was maintained in the study area from the first of January until the latter part of February, after which a steady decline in numbers was noted. The adult segment of the population began to decline after the middle of January. The maximum count of 38 adults was made on 16 January. On 14 March only 24 adults were seen. After this date all the adults present on the study area (the number varying between 14 and 16) can again be accounted for by assuming they are the breeding birds of the eight active nests of the area censused (Fig. lb). The population changes of the immature age-class varied only slightly from those of the adults. The arrival of the immatures on the study area probably coincides with that of the first adults, although the first birds were not actually noted on the study area until 7 November. From zero to two birds were noted THE WILSON BULLETIN June1964 116 Vol. 76, No.

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